Conductive pad and system for discharging static charges



March 19, 1951 s. E. LUNDEN 2,786,161

CONDUCTIVE PAD AND SYSTEM FOR DISCHARGING STATIC CHARGES Filed Jan. 11, 1954 3 51 I i v a1 lTrdQNexy United States PatentO CONDUCTIV E PAD AND SYSTEM FOR DIS- CHARGING STATIC CHARGES Samuel E. Lundeu, Palos Verdes Estates, Calif.

Application January 11, 1954, Serial No. 403,093

3 Claims. (Cl. 317--2) This invention relates to systems for discharging static charges from objects such, for example, as hospital equipment, which may and often do accumulate dangerous static charges by movement over floors. The danger attendant such charges has long been recognized and thus needs no extended discussion here. Various systems cmbodying conductive floorings and contact means for electrically connecting equipment to the flooring have previously been proposed, but none of these previous proposals have to my knowledge been completely satisfactory or reliable. The objects and accomplishments of my invention are the provision of a fully satisfactory and long-lived reliable system and, more particularly, a contact pad of lasting reliability, to insure the discharge of such static charges.

Briefly to outline a preferred and illustrative form of my invention, my new conductive pad comprises a brushlike element, composed of a material such as conductive rubber, or conductive vinyl plastic which is rubber-like in physical characteristics, and which has, over a considerable contact area, a multiplicity of flexible brushlike fingers each of which make a pressurable and wiping contact with the conductive floor. The existence of a multiplicity of such pressure contacts insures the necessary electrical connection to any kind of satisfactory conductive flooring, or, in fact, to any member, such for instance as a travelling belt, which itself accumulates a charge. But it particularly insures fully effective contact with the spaced conductive elements of my new flooring which is composed of tile, preferably a water-impervious ceramic, with regularly interspaccd small conductive elements which may be of any suitable conductive material but preferably of water-impervious conductive ceramic tile. On such a floor the contact area of the pad is made large enough to always cover one or more conductive floor elements; and in fact that each conductive finger of the pad acts individually to make pressure contact insures perfectly dependable electrical connection to whatever floor element or elements the pad is over.

Typical and illustrative forms of my new system and pad are illustrated in the accompanying drawings in which: i i

Fig. 1 is a schematic isometric view showing a frag ment of my new flooring and a typical form of my new contact pad mounted in a typical manner on the leg of a. piece of equipment and in contact with the flooring;

Fig. 2 shows a vertical central section of the illustrative form of my new contact pad, in contact with another type of flooring; and

Fig. 3 is'a section on line 3-3 of Fig. 2.

Fig. 1 shows the system in which my new contact pad, generally designated by the numeral 10, is applied in conductive contact with a conductive flooring of my new type. Although relative dimensions and the laying pattern may be varied, the following describes the flooring as it has been applied in actual use.

A conductive mortar setting bed 12 of Portland cement one part, sand four parts, and three percent (by weight of the dry cement) of acetylene black, is first poured and set. A conductive wire mesh 14 is embedded in the setting bed and is usually electrically grounded, although not necessarily so.

A conductive skim coat 16 of pure Portland cement with two percent acetylene black is then applied as a slurry. The tile flooring is then set on the skim coat.

In preferred laying design the tile fiooring consists of unglazed ceramic tile pieces 20, rectanguluar in shape and set in wickerweave pattern. That tile is non-conduc-v tive. In the square spaces left in that pattern small square conductive elements 22 are set on the skim coat, and the whole flooring is finally grouted. Conductive elements 22 may be of any suitable conductive material. For hospital use it is required that they, like the tile pieces, be impervious to water; and although they might be for example of any suitable metal or other suitable composition, they are preferably also of ceramic tile of a conductive composition. Such a conductive ceramic tile may be formulated in any suitable manner; a ceramic.

mix with a suitable proportion of finely divided conductive material, such as carbon, or a metal having the same coefficient of expansion as the ceramic, is illustrative.

The conductivity of the floor, measured between an electrode of 2 /2 inch diameter in contact with one or more of the conductive flooring elements and a ground connection, is made to be not less than 25,000 ohms. The conductivity of the floor between two electrodes placed three feet apart in contact with one or more of the conductive flooring elements 22 may then be somewhat rnore than that, but in no case should be over 1,000,000 ohms.

In the typical floor design as applied in actual use tile pieces 20 are 1 by 2%; inches in superficial dimensions, and the conductive elements 22 are one-half inch square. Other designs in actual use include various patterns of A" square tile with conductive tile of the same size spaced as desired; also alternate strips of nonconductive and conductive tile and variations thereof.

in the typical design a contact pad of about two inches diameter will always be in contact with at least one element; but to make good contact certain I prefer, for such a flooring, to make the contact pad about three inches in diameter so that it will then be always in contact with at least three elements.

My preferred form of contact pad is shown in detail in Figs. 2 and 3. As there shown it consists essentially of a fingered block base 30, which, and particularly its fingers 31, are composed of the flexible conductice rubber or vinyl or similar material. The base is molded about a stifi metal disk 32. Both the base and disk are preferably although not necessarily circular in plan, and, assuming the base to be three inches in diameter, the metal disk is about, say, 2% inches in diameter so as to give the base good pressure support over its whole area. A

. threaded nut 34 is welded rigidly to the center of the upper face of the disk and a threaded mounting rod 36 is threaded into the nut and locked in any desired adjusted position with a lock washer 38 and locking nut 40. The upper end of mounting rod 36 may be rigidly mounted in any suitable manner on the piece of equip ment that is desired to be protected, and height adjustment of the pad may be effected at the nuts 34 and 40. However, it is preferred to also thread the upper end of rod 36 and provide it with a pair of nuts 42, by which the rod may be mounted on the equipment with vertical adjustability.

Rod 36 may be mounted directly on some conductive (usually metal) part of the equipment, or a mounting bracket such as shown in Fig. 1 may be provided. As shown there the mounting rod is mounted in vertical ad justability on the end of a substantially rigid metal bracket Patented Mar. 19,1957

44 which is rigidly secured to the conductive metal leg 46 "of the appliance, the leg betng su ported on the usual caster 48 which is usually preferred to be rubber-tired and not always suitably conductive. It will be understood, of course, that the disk 32 and all the mounting parts 34, 36, 42, 44 are of conductive metal; so that the whole appliance (in hospital practice, usually of metal) that is supported on metal leg 46 is connected at low resistance to the pad 20.

A multiplicity of contact fingers 33., molded integrally with base 30, project from its lower face, the lower ends of these fingers being in a common plane. To give some illustrative figures from a contact pad that has proven fully satisfactory in practical use: the fingers may be about /8 inch long and V; inch diameter. They are spaced closely (see Fig. 3) over the whole lower surface of the base. The fingers particularly (and for convenience the base also) are molded of a material which has the elastic flexible qualities of a relatively soft rubber so that when the pad mounting is adjusted to put a pressure of say a pound or so on the pad with the fingers pressing on the floor, the fingers will flex and have a wiping action on the fioor when the appliance is moved. The individual flexibility of the fingers-their individually independent action-causes each of them to make good electrical contact with a floor surface even though that surface, as is usually the case, is not perfectly flat. The result is to make perfectly dependable electrical contact with the spaced conductive elements of my new ceramic flooring.

The contact pad will of course operate also with full effectiveness on other types of conductive flooring, such for instance as the one shown in Fig. 2. There the flooring is composed of a conductive cementitious base 50 which may have an embedded conductive metal mesh 52, surfaced for example with a conductive terrazo, or other conductive surfacing material 54, laid either as a homogeneous or continuous layer or in the form of tile. Or the flooring may consist of such surfacing as conductive rubber or linoleum without the conductive base of Fig. 2. The pad operates equally well on all such floorings. And the pad will operate equally Well on floors which have spaced conductive metal strips laid in the surface; the pad being of a size to always contact one or more of the spaced strips. To insure contact With narrow strips the fingers may be spaced closer together. For instance, in the pattern of Fig. 3, the fingers, or their lower wiper ends, may be made larger so as to form larger and more closely spaced wiper feet which may be made to have contact faces of any shape desired.

in the form of my invention now in practical use, the conductivity of the vinyl of the pad is in practice about 50,000 ohms when measured between the supporting bolt and a metal sheet in contact with the bottoms of the fingers. The total resistance to ground on a conductive flooring of approved resistance may thus be, say 100,000 ohms or more. but the pad itself has high enough resistance as to eliminate the possibility of discharge shock if the pad should chance to contact any grounded article. That resistance may of course be varied to controllably suit varying situations.

In the present practical use of my contact pad, it is secured in any suitable or convenient manner to the ap pliauce which is usually independently supported on the floor, as by casters. The pad mounting is then adjusted so that a small pressure is placed on the pad pressing it down on the floor surface. Another way of putting it is to say that the pad mount is so adjusted that the lower ends of the fingers flexed by contact with the floor, if

unfiexed, would be, say, /5 inch or so below the floor level 'on which the appliance rests.

In that form of my contact pad it supports only a negligible part of the weight of any usual appliance. However, the pad may be so proportioned and designed that it may, in multiple, be used as supporting feet for an appliance as well as for making electrical contact. By making the fingers larger in diameter, say inch, and molding them of a somewhat stiffer material (say medium rubber) the pads will support a considerable weight with the fingers flexing slightly. The usual sup porting feet or casters of the appliance may then be dispensed with and my pads substituted. As supports the pads have the added quality of insulating the supported appliance from vibration. The flexible fingers of my pads have the additional desirable advantage of being able to pass freely over raised surfaces such as threshholds.

Wide variations may be made in various features of structure and design of my system without departing from my invention, which is limited only as defined in the following claims. For instance, as has been indicated, the pad may have any suitable size and shape and the wiper feet of the flexible fingers may have any desired size and shape. The invention is of course not limited to use in hospitals but is useful in many other situations; for instance in industry and in machines, to contact travelling belts, tables, etc. instead of floors to discharge static therefrom. And in such cases the pad mounting may be grounded.

I claim:

1. A system for discharging static charges from appliances or the like movable over a floor, comprising in combination a flooring which has resistive conductivity from its surface to ground, a fingered contact pad comprising a conductive base and a plurality of elastically flexible fingers projecting from and spaced over the base and composed wholly of a material having resistive conductivity, the free ends of said fingers lying in a common plane and spaced in that plane over a contact area having susbtantial extent in both of two dimensions, and means for mounting and permanently electrically connecting the base of the conductive pad on and with and appliance or the like with the free ends of the fingers pressed into elastic frictional contact with the floor surface with the fingers flexed.

2. The system defined in claim 1 and in which the flooring has spaced resistively conductive surface elements, and in which the contact area of the pad fingers is such that in any position on the floor the fingers will engage at least one of such conductive elements.

3. A conductive pad for establishing electrical connection between an appliance and a floor for the discharge of static charges, said pad comprising a conductive block base, spaced flexible conductive contact fingers projecting from one face of the base and with their free ends mutually spaced in a common plane, said base and fingers being formed integrally of a flexible and resistively conductive rubber-like substance, and a low-resistance conductive mounting member comprising a conductive disk completely embedded in the base and substantially co-extensive with the fingered face of the base.

References Cited in the file of this patent UNITED STATES PATENTS 619,896 Lee Feb. 21, 1899 1,709,779 Crothers Apr. 16, 1929 2,426,299 Marick Aug. 26, 1947 2,457,299 Biemesderfer Dec. 28, 1948 

